1. Field of the Invention
The present invention generally relates to an image forming apparatus, and specifically relates to a charging mechanism for photosensitive drums.
2. Description of the Related Art
A tandem-type full-color electrophotographic image forming apparatus includes a plurality of toner-image forming units. Each toner-image forming unit includes a photosensitive drum on which an electrostatic latent image is formed, and a charging unit that electrically charges the photosensitive drum such that the electrostatic latent image on the photosensitive drum is developed with a single-color toner, such as yellow, cyan, magenta, and black, into a single-color toner image. The charging unit is arranged to abut against or remain detached from the photosensitive drum. To downsize such an image forming apparatus, the charging unit is usually arranged such that a charging roller, a charging brush, or a charging blade in the charging unit abuts against the photosensitive drum or lies adjacent to but detached from the photosensitive drum.
Each of the charging rollers charges a corresponding photosensitive drum either by using direct current (DC) charging or alternate current (AC) superimposition charging. The DC charging has a drawback that sometimes the photosensitive drums are not charged uniformly thereby failing to maintain the image quality. The AC-superimposition charging is advantageous in that the photosensitive drums are always uniformly charged thereby improving the image quality. That is why the AC-superimposition charging is gaining popularity. However, because the amount of discharging in the AC-superimposition charging is more, the surface of the photosensitive drums may get damaged due to the byproducts produced due to the discharge and the image quality may deteriorate.
In recent years, organic photosensitive drums are widely used in the image forming apparatuses. However, the organic photosensitive drums have a low degree of hardness and a weak tensile strength. Thus, they are prone to abrasion wear. Moreover, toner components or paper dust easily gets attached on the surface of such organic photosensitive drums thereby degrading the quality of cleaning or efficiency of image transfer, which results in corrupt images. In an image forming apparatus in which cleaning of the organic photosensitive drums is performed by blades, because of the high friction coefficient of the organic photosensitive drums, noise is generated at the time of cleaning. To solve such problems, a lubricant in liquid form or solid form, which also functions as a surface protectant, is applied on the surface of the organic photosensitive drums.
The lubricant lubricates the surface of the photosensitive drums such that the friction between the cleaning blades and the photosensitive drums is controlled. The lubricant also guards the surface of the photosensitive drums from being damaged due to collision of excited electrons generated during the AC-superimposition charging.
A tandem-type full-color image forming apparatus is disclosed in Japanese Patent No. 3587094 in which a photosensitive drum that forms electrostatic latent images in black is charged by a non-contact charging mechanism, while photosensitive drums that form electrostatic latent images in a color other than black are charged by a contact charging mechanism. In the contact charging mechanism, charging rollers are arranged to abut against the corresponding photosensitive drums.
A tandem-type full-color image forming apparatus is disclosed in Japanese Patent Application Laid-Open Nos. 2002-156806 and 2002-341618 in which photosensitive drums are charged by using charging members that abut against the corresponding photosensitive drums or lie adjacent to but detached from the corresponding photosensitive drums. The photosensitive drum that forms electrostatic latent images in black is charged by the DC charging, while the photosensitive drums that form electrostatic latent images in a color other than black are charged by the AC-superimposition charging.
A tandem-type full-color image forming apparatus is disclosed in Japanese Patent Application Laid-Open No. 2001-034111 in which the amount of lubricant applied on the surface of each photosensitive drum is adjusted as per the requirement. Particularly, more amount of lubricant is applied on the photosensitive drums that lie on the downstream along the direction in which a transfer paper is conveyed.
A tandem-type full-color image forming apparatus is disclosed in Japanese Patent Application Laid-Open No. 2004-061855 in which the amount of lubricant applied on the surface of a photosensitive drum that forms electrostatic latent images in black is more than the amount of lubricant applied on the surface of photosensitive drums that form electrostatic latent images in a color other than black.
In the tandem-type full-color image forming apparatus as disclosed in Japanese Patent No. 3587094, because the photosensitive drums that form electrostatic latent images in a color other than black are charged by the contact charging mechanism, the surface of those photosensitive drums is damaged due to the friction with the charging rollers. As a result, such photosensitive drums have less durability than the photosensitive drums that are charged by a non-contact charging mechanism. Moreover, toner gets attached easily to such a contact charging mechanism thereby reducing its durability. Instead of the contact charging mechanism, if a corona charging mechanism, particularly a scorotron charging mechanism, is used to charge a photosensitive drum, then poison gases such as ozone are produced that need to be properly disposed. Hence, if the scorotron charging mechanism is used to charge all the photosensitive drums, the size of the image forming apparatus becomes large because of the additional components required to exhaust the poison gases.
On the other hand, in the tandem-type full-color image forming apparatus as disclosed in Japanese Patent Application Laid-Open Nos. 2002-156806, 2002-341618, 2001-034111, and 2004-061855, the problem of damaging the surface of the photosensitive drums due to the friction with the charging members is not comprehensively solved. Hence, there is a need of a technology that would uniformly charge the photosensitive drums with minimum damage to their surface.